In recent years, batteries have been the focus of attention as power supplies for small portable devices such as smart phones, electric automobiles, and hybrid automobiles. Lithium ion batteries, which have a small volume and a large capacity, are presently a particular focus of attention among batteries.
Technical demands for batteries include reducing size and weight, increasing the degrees of freedom of the shape, and enhancing safety. As a means capable of meeting these demands, electrolyte solidification using an inorganic solid electrolyte, a polymer electrolyte, a gel electrolyte, or the like as the electrolyte constituting a battery is being researched. In electrolyte solidification, using a gel electrolyte is presently the most promising from the perspective of ionic conductivity.
Although gel electrolytes have relatively good ionic conductivity, there are demands for improvements. In order to achieve higher ionic conductivity in a gel electrolyte, it is necessary for the gel electrolyte to hold a larger amount of a non-aqueous electrolyte solution. However, when the gel electrolyte contains a large amount of a non-aqueous electrolyte solution, the gel strength decreases, so achieving both ionic conductivity and gel strength is a critical issue.
Under such circumstances, copolymers containing vinylidene fluoride (VDF) and hexafluoropropylene (HFP) or chlorotrifluoroethylene (CTFE) (for example, see Patent Document 1), vinylidene fluoride copolymers in which at least one type of a structure formed by esterifying some or all carboxyl groups or carboxylic acids and an acetic anhydride structure (for example, see Patent Document 2), copolymers containing vinylidene fluoride and unsaturated dibasic acid monoesters (for example, see Patent Documents 3 and 4), and the like have been proposed, but a gel electrolyte which sufficiently achieves both ionic conductivity and gel strength has not yet been obtained.